An electronic device typically comprises a semiconductor material chip on which one or more electric components are integrated, and a package wherein the chip is encapsulated for protecting it and for allowing access to terminals thereof.
In this respect, the package typically comprises an insulating body (for example made of resin) having exposed metallic leads, each one of which is electrically connected to a corresponding conductive terminal of the chip (for example, through a wire-bonding technique). The package leads are used for connecting the package (and hence the corresponding conductive terminals of the chip) to external circuits. For such purpose, the electronic device is usually mounted on a Printed Circuit Board (PCB), for example, by Surface Mounting Technology (SMT), wherein the package leads comprise flat pads that are fixed to corresponding conductive tracks of the PCB by means of a slight pressure (pick and place), and then reflowed on the same, or by Through-Hole Technology (THT), wherein the package leads comprise reophores that are inserted into through holes of the PCB and back-welded on it.
An electronic device for power applications (e.g., power supplies, DC to DC converters, and motor controllers), hereinafter simply referred to as a “power device”, comprises one or more power components integrated in the chip. A very common class of power components is for example represented by MOS power transistors; the latter, being affected by high voltages (such as for example 5.5 V-850 V), are subjected to considerable heating during operation thereof. For this reason, the chip on which the power component is integrated needs a package that, in order to ensure adequate heat dissipation properties (so as to avoid the occurrence of overheating capable of causing chip malfunctioning or breakages), is provided with one or more heat-sinks for dissipating the heat generated by the chip during operation thereof to the outside.
A simple but effective heat-sink suitable for being employed in packages of power devices comprises one or more metal plates, also referred to as “heat slugs”, each one contacting (at least a portion of) a surface of the chip. For example, the package may be provided with a heat slug extending between a surface of the chip and a mounting surface of the insulating body facing the PCB.
A typical industrial process for manufacturing a plurality of power devices comprising heat slugs provides for the execution of the following sequence of operations.
Making reference to SMT power devices (similar considerations may be however applied to THT power devices), the first operation involves the use of a common support structure (referred to as a “leadframe”) in conductive material, for example copper, comprising for each power device to be manufactured a support cell for the chip of said power device, comprising a corresponding heat slug and corresponding junction sacrificial portions surrounding the heat slug. Lead blocks (precursors of the leads in the electronic devices) extend from the junction sacrificial portions towards the heat slugs. In the support structure, the heat slugs, the junction sacrificial portions and the lead blocks corresponding to the plurality of power devices are connected together to form a common single body (leadframe).
The next operation provides that for each power device to be manufactured a semiconductor material chip integrating the power components (e.g., a MOS power transistor) is mounted on a first surface of the corresponding heat slug. In jargon, this operation is called a “die attach”.
The conductive terminals of each chip are then electrically coupled to ends of the corresponding lead blocks in the support structure, for example using interconnection wires, having circular cross-section, or interconnection twin leads, having rectangular cross-section, in conductive material. In jargon, this operation is called “wire bonding”.
The next step provides for forming the insulating body of each power device by encapsulating the corresponding chip, the corresponding heat slug, and portions of the corresponding lead blocks into a corresponding insulating body, in such a way that at least a portion of a second surface of the heat slug opposite the first surface is exposed from a face of the insulating body (the mounting surface). This operation can be for example performed by injection molding of resin material on the support structure. This operation is called in jargon a “molding” operation.
The next operation, called in the jargon a “cropping” operation, provides for the separation of the power devices from the common support structure, by sectioning the lead blocks.
Similar considerations apply if the power devices are provided with a heat slug extending between a surface of the chip and a free surface of the insulating body opposite the mounting surface.
When a power device of the abovementioned type is subjected to thermal and/or mechanical stresses, such as for example during the operations directed to fix the power device to a PCB, the resin forming the insulating body may detach from the heat slug, causing delamination at the interface between the resin of the mounting surface of the insulating body and the borders of the exposed second surface of the heat slug. If the delamination propagates and reaches the chip, a crack may correspondingly form in the latter. Said crack in the chip may propagate in turn until reaching the active portion of the chip (i.e., the portion thereof wherein the components are located), causing a partial or complete breakdown of the power device.
In order to reduce the resin-heat slug detachment occurrences, known solutions provide for shaping side borders of the heat slug in such a way to exhibit a pronged profile comprising recessed and protruding portions, and for forming grooves in the first surface of the heat slug. In this way, during the molding operation directed to the formation of the insulating body, resin fills the grooves and the recessed portions of the profile of the heat slug, causing the insulating body and the heat slug to be interlocked. This results in an increased overall adhesion between the insulating body and the heat slug.
It has been noted that the increased adhesion between the insulating body and the heat slug obtainable with the abovementioned known solutions may still not be sufficient to avoid that resin forming the insulating body detach from the heat slug when the power device is subjected to high thermal and/or mechanical stresses. Indeed, even with the presence of the pronged profile in the side borders of the heat slug and with the presence of grooves in the first surface of the heath slug, if the power device is subjected to severe thermal and/or mechanical stresses, a delamination at the interface between the resin of the mounting surface of the insulating body and the borders of the exposed second surface of the heat slug may still occur, and such delamination may still be able to propagate toward the chip, causing the formation of a crack in the latter.
There is a need in the art to provide an electronic device which is able to sustain also high thermal and/or mechanical stresses without causing possible delamination to reach the chip.